The present invention relates to a rotary screw compressor for a gaseous working fluid comprising a male rotor and a female rotor mounted in a causing composed of a high pressure end section, a low pressure end section and a barrel section extending therebetween, said casing forming a working spaced generally in the shape of two intersecting parallel bores surrounded by barrel and end walls, each of said rotors having helical lobes and intermediate grooves through which the rotors intermesh forming chevron-shaped compression chambers in said working space, each of said bores housing one of said rotors, said casing being provided with an inlet port and an outlet port, each of said rotors being provided with shaft extensions mounted in bearings in said end sections and extending into first chambers in the low pressure end section and into second chambers in the high pressure end section, said low pressure end section having means for supply of liquid to aid first chambers and said high pressure end section having means for supply of liquid to said second chambers.
In compressors of this type the liquid, e.g. oil, supplied to the chambers in the end sections for bearing lubrication and other purposes usually has been drained to the low pressure channel of the compressor, as shown for instance in U.S. Pat. No. 3,314,597.
As the oil drained from the chambers in the end sections circulates within the compressor plant and gets a maximum temperature corresponding to the temperature of the working fluid in the high pressure channel, it has to be cooled down before recirculation into the compressor. However, owing to the temperature of the available cooling fluid and the practically possible size of the cooler, the oil introduced into the compressor will have a considerably higher temperature than the temperature of the working fluid to be compressed. The contact between the working fluid and the oil of the higher temperature during the inflow phase result in a healthy of the working fluid and thus is a decrease of the volumetric efficiency. There is also a considerable power required for the inflow of the oil from the low pressure channel through the low pressure port into the working space. Furthermore, a certain amount of the oil flows through the bore of the male rotor and has to be accelerated to the high speed of the tips of the lobes thereof.
A special problem arises in compressorsm forming a part of a refrigeration cycle using a working fluid of the type being dissolvable to a considerable extent in the oil, such as fluids of the type normally referred to as Freon, and commercially known for instance as R-12 and R-22. The oil supplied to the chambers in the end sections for bearing lubrication, shaft sealing, thrust balancing and similar purposes, normally has a pressure exceeding the presence in the high pressure channel of the compressor and the amount of working fluid dissolved therein is considerable. When the chambers are drained to the low pressure channel most of the working fluid is evaporated out of the oil as the solubility decreases with decreasing pressure. The amount of working fluid in this way supplied to the low pressure channel is so large that it will need a very considerable portion of the displacement volume of the compressor. The same amount of working fluid is during the compression dissolve in the oil. Owing to this fact the amount of working fluid passing through the compressor and circulating within the complete cycle will be mush less than the nominal capacity of the compressor or in other words thee volumetric efficiency of the compressor will be low.
All of the factors mentioned above will be more accentuated the smaller the dimensions of the compressr are as the amount of oil supplied to the chambers in the end sections cannot be reduced in the same proportion as the reduction of the amount of working fluid passing through the compressor.
U.S. Pat. No. 3,462,072 discloses a rotary screw compressor in which the above described problems are avoided in that the chambers in the high pressure end section are drained not to be the low pressure channel but to the working spaced of the compressor through an opening in the wall of the working space. In the embodiment shown in FIG. 3 also the chambers in the low pressure end section are drained to the working space through this opening. Although this construction avoids the problems discussed above it can only be satisfactorily used when the pressures in the bearing chambers at each side are of about the same level. As often is the case, the pressure in the chambers in the high pressure end section is higher than that in the chambers in the low pressure end section. When these pressures are short circuited through the drainage system there is a risk that high pressure oil will flow into the chambers in the low pressure end section.
British Patent No. 1,599,416 discloses another example of draining the bearing chambers. The bearing chambers in the high pressure end section are connected through a channel with the gear box and the oil from the chambers in both end sections is then drained from the gear box to the working space through a common opening in the barrel wall. The oil from the chambers in the high pressure end section thus has to circulate through the sump of the gear box and the construction requires special connections for this.
Swedish Patent No. 438 184 discloses still another drainage system, in which the bearing chambers in the high pressure end section are drained to a compression chamber in the working space, whereas the oil from the bearings in the low pressure end section together with the oil from the gear box is collected in an oil sump. Since the sump is located beneath the compressor, the oil from the sump cannot be drained to a compression chamber or the suction channel. It is therefore drained to an expanding chamber formed by the rotors, before this chamber is brought into communication with the suction port and begins to be filled with air. The vacuum thereby created is enough to suck the oil from its lower level. This system is of a very special design and if it was to be used in cases where the oil pressure in the chambers in the low pressure end section exceeds the inlet pressure conditions the drawbacks initially discussed would occur.
The object of the present invention is to improve the oil drainage system of a type similar to that disclosed in U.S. Pat. No. 3,462,072 and accomplish oil drainage from the bearing chambers in the two end sections in a new and better way.